Pluto Reclassification Could Change Number of Solar System's Planets

For decades, it has consisted of nine planets, even as scientists debated whether Pluto really belonged. Then the recent discovery of an object larger and farther away than Pluto threatened to throw this slice of the cosmos into chaos.

Should this newly found icy rock known as 2003 UB313 become the 10th planet? Should Pluto be demoted? And what exactly is a planet, anyway?

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Ancient cultures regularly revised their answer to the last question and present-day scientists aren't much better off: There still is no universal definition of "planet."

That all could soon change, and with it science textbooks around this planet.

At a 12-day conference beginning Monday, scientists will conduct a galactic census of sorts. Among the possibilities at the meeting of the International Astronomical Union in the Czech Republic capital of Prague: Subtract Pluto or christen one more planet, and possibly dozens more.

"It's time we have a definition," said Alan Stern, who heads the Colorado-based space science division of the Southwest Research Institute of San Antonio. "It's embarrassing to the public that we as astronomers don't have one."

The debate intensified last summer when astronomer Michael Brown of the California Institute of Technology announced the discovery of a celestial object larger than Pluto.

Like Pluto, it is a member of the Kuiper Belt, a mysterious disc-shaped zone beyond Neptune containing thousands of comets and planetary objects. (Brown nicknamed his find "Xena" after a warrior heroine in a cheesy TV series; pending a formal name, it remains 2003 UB313.)

The Hubble Space Telescope measured the bright, rocky object at about 1,490 miles in diameter, roughly 70 miles longer than Pluto. At 9 billion miles from the sun, it is the farthest known object in the solar system.

The discovery stoked the planet debate that had been simmering since Pluto was spotted in 1930.

Some argue that if Pluto kept its crown, Xena should be the 10th planet by default — it is, after all, bigger.

Purists maintain that there are only eight traditional planets, and insist Pluto and Xena are poseurs.

"Life would be simpler if we went back to eight planets," said Brian Marsden, director of the astronomical union's Minor Planet Center in Cambridge, Mass.

Still others suggest a compromise that would divide planets into categories based on composition, similar to the way stars and galaxies are classified. Jupiter could be labeled a "gas giant planet," while Pluto and Xena could be "ice dwarf planets."

"Pluto is not worthy of being called just a plain planet," said Alan Boss, an astrophysicist at the Carnegie Institution in Washington, D.C. "But it's perfectly fine as an ice dwarf planet or a historical planet."

The number of recognized planets in the solar system has seesawed based on new findings. Ceres was initially classified as a planet in the 1800s, but was demoted to an asteroid when similar objects were found nearby.

Despite the lack of scientific consensus on what makes a planet, the current nine — and Xena — share common traits: They orbit the sun. Gravity is responsible for their round shape. And they were not formed by the same process that created stars.

Brown, Xena's discoverer, admits to being "agnostic" about what the international conference decides. He said he could live with eight planets, but is against sticking with the status quo and would feel a little guilty if Xena gained planethood because of the controversy surrounding Pluto.

"If UB313 is declared to be the 10th planet, I will always feel like it was a little bit of a fraud," Brown said.

For years, Pluto's inclusion in the solar system has been controversial. Astronomers thought it was the same size as Earth, but later found it was smaller than Earth's moon.

Pluto is also odd in other ways: With its elongated orbit and funky orbital plane, it acts more like other Kuiper Belt objects than traditional planets.

Even so, Pluto remained No. 9 because it was the only known object in the Kuiper Belt at the time.

When new observations in the 1990s confirmed that the Kuiper Belt was sprinkled with numerous bodies similar to Pluto, some scientists piped up. In 1999, the international union took the unusual step of releasing a public statement denying rumors that the ninth rock from the sun might be kicked out.

That hasn't stopped groups from attacking Pluto's planethood. In 2000, the Hayden Planetarium at New York's American Museum of Natural History unleashed an uproar when it excluded Pluto as a planet in its solar system gallery.

Earlier this year, NASA's New Horizons spacecraft began a 9½-year journey to Pluto on a mission that scientists hope will reveal more about the oddball object.

The trick for astronomers meeting in Prague is to set a criterion that makes sense scientifically. Should planets be grouped by location, size or another marker? If planets are defined by their size, should they be bigger than Pluto or another arbitrary size?

The latter could expand the solar system to 23, 39 or even 53 planets.

It's not an academic exercise — the public may not be open to a flood of new planets. Despite their differences, scientists agree any definition should be flexible enough to accommodate new discoveries.

"Science progresses," said Boss of the Carnegie Institution. "Science is not something that's engraved on a steel tablet never to be changed."